Studying the Delaware River - 2010 Report

The Delaware estuary, New Jersey's largest estuary system, is a semi-enclosed body of water where freshwater from the Delaware River mixes with salt water from the Delaware Bay. The estuary is a migratory route for many recreational and commercial fish and provides critical spawning and feeding grounds and nursery areas for many species.

The success of a species is contingent upon the survival of their young. The Delaware estuary provides a suitable nursery environment for young fish to grow. Monitoring populations of these juvenile fish is essential for fishery managers to estimate abundance and evaluate the success of the population. These assessments provide a means to predict population trends and future harvest potential of monitored species.

Bureau of Marine Fisheries biologists within the New Jersey DEP's Division of Fish & Wildlife conduct several surveys each year to study the status of species populations within the estuary. One of these surveys is the Delaware River Seine Survey.

A unique aspect of this survey is its longevity - it has been conducted for 31 consecutive years. Data from such a large period of time is highly beneficial to species population studies. Not only does this survey tell us how many fish there are from year to year, but the data also contributes to the development of fisheries management plans and projections of sustainable harvest levels.

The survey area is divided into three regions. These regions have not changed since the beginning of the survey. Changes have occurred to the number of stations, station locations and dates sampled over the years.

All regions are tidal. Region 1 is the southernmost sampling area. It is a brackish region extending from the spring saltwater/freshwater interface to the Delaware Memorial Bridges. Region 2 is the central sampling area. It is a brackish to freshwater area extending from the Delaware Memorial Bridges to the Schuylkill River. Region 3 is the northernmost sampling area. It is a freshwater area extending from Philadelphia to the fall line at Trenton.

Currently, there are 32 fixed stations (beaches): eight in Region 1, 16 in Region 2 and eight in Region 3. Each station is sampled bi-monthly from mid-June to mid-November, resulting in 320 seine hauls each year.

Images show net placement and taking inventory of catch.Click on images to enlarge.

Survey methodology has remained fairly consistent through the years. The boat used for this survey is a 20-foot Privateer, Roamer skiff. This fiberglass boat has a side console and a 140-horsepower Suzuki 4-stroke outboard engine. The net used is a 100-foot long, 6-foot deep seine net with ¼-inch mesh that has a bag in the middle where the fish are collected.

One end of the net is held close to shore by a crewmember on the beach while the rest of the net is set off the bow of the boat as it backs away from the beach. The boat sets the net with the current, before turning back towards the beach to form a "U" shape. To complete the haul, the net is pulled onto the beach from both ends and the catch is funneled into the bag in the center of the net

After a seine haul is complete, and the net has been pulled onto the beach, all fish captured are sorted by species, counted and sub-samples of target species are measured. In addition to striped bass, target species include white perch, American shad, bay anchovy, Atlantic croaker, weakfish, blueback herring, Atlantic menhaden, Atlantic silverside, alewife, spot, blue crab, bluefish, hickory shad, winter flounder, black drum and summer flounder.

Since the survey's inception, sampling crews have set a beach seine 6,704 times and caught 1,304,648 fish. Fish & Wildlife has averaged 194 fish per haul since 1980, with 99 different species identified. The five most abundant species caught include blueback herring, Atlantic menhaden, bay anchovy, white perch and American shad. The primary target species (striped bass) is the eleventh most commonly captured.

Menhaden and Striped Bass - In 2003, the average number of striped bass caught per haul was 12 fish, a record year for this survey. The striped bass fishing moratorium in the 1980's allowed striped bass populations to bounce back rapidly and bass numbers have continually increased since the early 1990's. On the other hand, while striped bass numbers are increasing, the average numbers of Atlantic menhaden are decreasing. This development concerns biologists because menhaden are a key food source for striped bass. It is possible that the menhaden population decrease is a direct result of the increasing striped bass population. Closely monitoring menhaden and striped bass numbers over the next few years will be helpful in understanding this predator-prey dynamic and potential management implications for both species.

Blueback Herring and American Shad - Blueback herring and American shad numbers have also increased overall since the beginning of the survey. However, during years of severe flooding (2006) and severe drought (2002), catches tend to be lower than normal. In 2001, the average number of blueback herring caught per haul was 98 but in 2002, the average dropped to six fish per haul. Likewise, American shad averages dropped from 21 fish per haul in 2001 to one fish per haul in 2002. Both species seem resilient, as they bounced back with abundance in alternating years.

Unfortunately, due to flooding in 2006, blueback herring and American shad population counts dropped to all-time lows of less than one fish and two fish per haul, respectively. But once again, American shad showed their resiliency in the Delaware River during 2007, with the highest abundance recorded at 37 fish per haul. Blueback herring abundance remained low (25 fish per haul) for 2007, causing some concern for this species. However, an increase to 33 fish per haul occurred in 2009.

White Perch - The abundance of white perch has also increased over the survey years. The average per-haul number increased substantially from 1990 to 1991, with average numbers caught jumping from 7 fish per haul to 32 fish per haul. To date, the best year was 1996 when white perch numbers reached an average of 67 fish per haul. This was a productive year for many other species as well. Since 1996, the number of white perch has fluctuated without trend showing an additional peak in 2003.

Bay Anchovy - The bay anchovy is a species whose numbers are decreasing at an alarming rate. These fish are usually one of the most abundant species in the Delaware estuary and are a primary food source for many fish inhabiting the river, including weakfish, bluefish and striped bass. The average number caught per seine haul in this survey has been declining since 2000. Bay anchovy data correlates well with data from Fish and Wildlife's Finfish Trawl Survey in Delaware Bay, which also indicates a bay anchovy decline since 1998.

The potential impact from this decrease has yet to be recognized, but the recent decline in the Delaware Bay's weakfish population may be an indication of a food chain imbalance within this estuary. Continued monitoring of the estuary's bay anchovy population is important. Fish and Wildlife will continue to conduct research and analyze data to assess the status of the anchovy population and relationships to prey fish populations.

Sturgeon - Historically, the Delaware Estuary has been an important habitat for two species of sturgeon: the shortnose sturgeon, and its cousin, the Atlantic sturgeon. Atlantic sturgeon primarily live in the ocean, and migrate through the estuary to spawn in freshwater. Shortnose sturgeon spend most of their time in the brackish water of the estuary, moving upstream to fresher water to spawn.

Over the duration of the Delaware River seine survey, only one sturgeon has ever been caught. In August 2004, a shortnose sturgeon was caught during a haul at the Oldman's Point station just down river from the Commodore Barry Bridge. The young-of-year sturgeon measured only 4.6-inches.

Water quality parameters such as salinity, water temperature and dissolved oxygen are recorded at every station with a handheld dissolved oxygen (DO) meter that simultaneously measures several different water quality parameters. The meter has a probe attached to it with a long cord. This probe is lowered over the side of the boat and placed into the water, just below the surface. The DO meter gives readings within a matter of seconds and provides surveyors with fast, accurate measurements. Because all water quality parameters are influenced by many outside factors, it is necessary to record this data at the site of each seine haul.

Salinity is the saltiness, or dissolved salt content, of a body of water. In the Delaware River seine survey, it is measured in parts per thousand (ppt) which is the measure of grams of salt per liter of water. In a tidal estuary, there are many factors which influence salinity. Freshwater run-off from rainstorms lowers salinity levels, while droughts lead to an influx of saltwater from the ocean. In this survey, Region 1 is the southernmost area, closest to the Delaware Bay and has the highest salinity averages, ranging from 0 ppt to 12 ppt in 2010. Region 2 is a brackish water area, with averages ranging from 0 ppt to 3 ppt in 2010. While still tidal, Region 3 is almost completely freshwater. Survey averages have ranged from 0 ppt to 0.102 ppt in 2000. Overall, trends show the salinity of the Delaware River has remained fairly constant through the years.
Salinity Graph (pdf, 13kb)

Water temperature is also affected by many factors, including water depth, tides, and the weather. On average, the water temperature during this survey, measured in degrees Celsius, has increased over the years. The lowest average temperatures in all three regions occurred in the early 1990s. In Region 1 high averages were collected in 2007 measuring 23.9° C. In Region 2, it was 2001 at 23.8° C. Finally, Region 3 peaked out in 2005 at 24.0° C.
Water Temperature Graph (pdf, 13kb)

Dissolved oxygen (DO) is a measure of the amount of gaseous oxygen that is dissolved in water. It is measured in milligrams per liter (mg/L). Oxygen gets into the water by diffusion from the surrounding air. This happens through a number of methods including rapid water movement (tides, run-off or boat traffic) and air movement above the water (wind or storms). Trends show a gradual decrease in the overall average of DO in the Delaware River. Region 1 peaked out at an average of 7.867 mg/L in 1996 and bottomed out in 2007 at 6.478 mg/L. Region 2, had its highest average in 1990 at 7.378 mg/L and lowest in 2007 at 6.394 mg/L. Like Region 1, Region 3 had its highest average in 1996 at 8.01 mg/L and its lowest in 2001 with a value of 5.94 mg/L.
Dissolved
Oxygen Graph (pdf, 11kb)

Regions 1 and 2 are historical striped bass spawning grounds. Results of this seine survey in recent years have confirmed that these regions are just as important now as they were in decades past. In fact, data from this and similar surveys in other states have reflected an increase in the striped bass population along the entire East Coast. Surveys like this are just the beginning of the stock assessment process for many species.

Since the inception of the survey, the abundance of several species has declined including spot, American eel, channel catfish and weakfish. While it is not completely certain why these species populations are decreasing, future research is planned to determine the underlying causes.

Seine surveys, as with all fishery surveys, are important for ecosystem management. Not only do they provide information on species abundance, but they also provide a broader source of data on interactions with other species and associations with environmental factors. Without these surveys, biologists could not identify species interactions that might predict future fishery management needs. With consistent monitoring along the entire New Jersey coast, we can identify declines in species abundance and implement management actions before it's too late.

Anglers have an important opportunity to help biologists collect valuable data through the Division's Recreational Saltwater Angler Survey. Interested anglers are encouraged to log on at www.njfishandwildlife.com/marinesurvey.htm to enter information about personal fishing trips. All information collected helps improve the management of our marine resources and the sustainability of New Jersey's marine fisheries.